Vehicles now have a CAN buss that monitors the loads. For rear lights, there is one power lead, the CAN buss tells the lights to come on, this is supposed to save wire weight. Pierce had so much trouble on fire apparatus that LA County will not accept any apparatus with it.
Question: did you supply the chassis or did Osage? It may be possible that you were supplied a non-police -fire setup and got a regular chassis. There is a difference in wiring, computer shielding and location of certain parts. You cannot buy a public safety rated chassis off of the lot, I tried. That is a fleet option only. Only way is if the ordering agency refuses to take delivery because it was not what they ordered.